1. Field
The present invention relates generally to wireless receivers, and more specifically to finger assignment in rake receivers based on signal path concentration.
2. Background
In wireless communication systems, a signal transmitted from a transmitter is often subjected to dispersion, reflection, and fading resulting in multiple versions of the signal arriving at the receiver at different times. In direct sequence spread spectrum systems, rake receivers receive and combine the multiple time-shifted signals to receive the original transmitted signal. A conventional rake receiver includes multiple fingers where each finger includes a correlator synchronized to receive one of the time-shifted signals. A repeating pseudorandom code is applied to the incoming signal such that the bits of the pseudorandom code are aligned with the corresponding bits of the incoming signal. In order to assign each finger to a different signal of the time shifted versions, a searcher identifies the signal paths from the transmitter to the receiver. A pilot channel is often observed by the searcher to determine the time relationships between the multiple versions of the signal arriving at the receiver. In some situations, however, the searcher is not able to identify all of the paths in a short time. For example, time is often limited in identifying signal paths when user equipment (UE), such as an access terminal, comes out of sleep mode. In code division multiple access (CDMA) systems, the access terminal must wake up from a sleep mode periodically to demodulate a paging indicator channel to determine if an incoming call is arriving. In order to maximize battery life, the time that the access terminal is not in sleep mode is minimized resulting in a limited time for the searcher to identify the signal paths. In high speed fading scenarios, the searcher may not identify all useful signal paths in the time allowed.
Therefore, there is a need for rake finger assignment during high speed fading scenarios.